Power controller circuits presently exist which provide automatic sensing of a primary power source voltage and couple the primary power source voltage to an electrical load when the primary power source voltage is above a predetermined threshold voltage and which switch a backup battery to the electrical load when the primary power source voltage is below the threshold. Some of these power controllers have been embodied in CMOS integrated circuits which utilize MOS transistors to couple the primary power source to the electrical load or the backup battery voltage to the electrical load.
A problem which may develop with respect to power controllers with lithium energy cells as backup batteries is that the lithium energy cells are sensitive to charging currents and the lithium cells tend to fail if the cells are recharged even by a small amount.
As will be discussed more thoroughly below with respect to FIG. 2, it is possible, when the primary power source has failed, to charge the lithium cell through the MOS transistors via a signal line to the electrical load which couples through the electrical load a voltage greater than the battery voltage which in turn is impressed upon the output voltage terminal of the power controller to thereby transfer charge into the lithium energy cell backup battery.
Therefore, it can be appreciated that a CMOS integrated circuit power controller circuit which protects against charging of the backup battery is highly desirable.